The present invention relates to a method of repairing turbine blades and is primarily but not exclusively concerned with the repair of blades where it is required to carry out a modification or a repair to a blade in the end region thereof.
Turbine blades may be used, for example, in steam turbines and are subjected to a harsh environment which after a while causes wear and damage to the blades.
The outer end of the turbine blades in a rotor assembly is the part of the blade which has the greatest linear speed in use and hence is the part that is most prone to damage, in particular erosion from water droplets in the steam.
It is fairly common for turbine blades, arranged in a rotor for example, to have a shroud extending around the outer ends of the blades, each blade being provided with at least one tenon at the outer end adapted to co-operate with the shroud in a manner such that each blade is secured to the shroud.
It is also common for there to be lacing wires extending through holes from one blade to another to tie together the blade, or at least groups of blades in a turbine blade array or as a still further alternative, the blades may be provided with snubbers adapted to contact each other, all the above mentioned features such as shrouds, and brazed lacing wires which have the effect of making the array as a whole more rigid and to prevent relative movement occurring between the blades in the array, or snubbers and "loose" lacing wires which reduce vibration amplitudes by friction clamping.
Turbine blades provided with shrouds may inter alia suffer from crevice corrosion. Crevice corrosion is caused by entrapment of salts carried by the steam in, for example, crevices where a shroud hole and a tenon on the blade is loose thus enabling a crevice to occur in which salt may accumulate or alternatively if the blades are provided with lacing wires, such crevices may exist in the holes in the blades or in joints in the wire itself.
Blades at the low pressure end of the turbine are also subjected to water impact in view of condensation on stator blades or nozzles which eventually causes the formation of large water droplets which subsequently break away from the stator blades or nozzles and causes substantial impact on the leading edge regions of the rotor blades. Since it is the outer end of the rotor blade that has a far greater linear speed, the damage will be far greater at the outer end.
Blades at the low pressure end of the turbine are frequently subjected to the effect of "stress corrosion cracking" which is prone to occur around discontinuities in the turbine blades, such as lacing wire holes and for example the attachment of the blade to a shroud, which attachment may be a tenon provided on the end of the blade and a through bore provided on the shroud.
The provision of new repair techniques by the applicant has enabled the satisfactory repair of turbine blades rather than the total replacement of the blade, this considerably reduces the cost of the repair and decreases the down time of the turbine and providing the repair gives satisfactory service life is hence to be preferred.
It has been proposed before by the applicant that it is unwise, where an end part of a turbine blade has to be replaced to effect the join of the new part immediately adjacent any discontinuity in the blade or adjacent, for example, an attachment of the blade to a shroud, e.g. a tenon and it is preferred to displace a join of a new part to the blade by a small distance so that the area subjected to most stress in use, for example the attachment of a tenon on the end of a blade to a shroud, is made from parent metal and not cast metal that would occur if for example a tenon was built up by a welding operation.
The position of a join of a new end piece to a turbine blade is critical since inevitably the join will constitute a critical point on the blade which, through the nature of the weld material, may not be as strong as the parent material of the blade.
It is desirable therefore that the position of the join is selected so that it is not near any discontinuity in the blade, such as a lacing wire hole and is not immediately adjacent any other position of stress concentration such as the tenon on the end of the blade.
It has been found that in use of the blades, whereas the effects of crevice corrosion cracking and general erosion are well known, other stresses applied to the blade are not significant. However, it has been found that turbine blades are at a certain time in their use subjected to a critical stress, for example at its operational speed or during the acceleration of a turbine from an at rest or steady slow angular velocity at which it is substantially at rest to its operational speed.
Each blade if untied or unconnected to adjacent blades will exhibit a number of resonant frequencies, and these frequencies are altered and indeed some modes of vibration are minimised by the inclusion of cover bands, shrouds, lacing wires, snubbers etc., and the frequency at which the blade may have otherwise been excited to vibrate at an in-use speed or operational speed of a turbine are thus minimised so that there is no undue resonance in a blade at an operational speed.
The desired speed of operation of a turbine however and the length of the blades normally results in there being speed through which a turbine must pass from its at rest speed to an operational speed at which the blade would be excited to vibrate and which causes resonance in the turbine blade and of course subjects them to considerable stress. This resonance is, as aforementioned, minimised by the provision of cover bands, lacing wires, etc., and is of little consequence.
However, applicant has found that problems do occur and damage to turbine blades can be severe if for example one blade breaks free from a cover band because, for example a tenon has broken or become eroded, and the blade is substantially untied to the remainder of the blade, apart from the securing of the blade to a rotor by its root. This permits the blade to resonate at its natural frequency, depending upon its mass and its length, at its operational speed or during acceleration or deceleration of the rotor from or to its operational speed from an at rest speed.
The natural frequency of vibration of the blade, as aforementioned determined at least in part by its mass and its length, will result in parts of the blade at positions along its length having a considerable amplitude of vibration which causes stresses in the blade far more severe than the stresses that would occur during normal use.
It is an object of the present invention to provide a new or improved method of repair to a turbine blade.